INTRODUCTION: The sex ratio at birth (usually denoted as M/T=male divided by total births) is influenced by cultural factors. The Chinese Zodiac holds that women born in the Fire Horse year (1966) would be unlucky. It has been shown that significantly less children were born in Japan in this year. This study was carried out in order to ascertain whether this year was also associated with any change in M/T.
METHODS: Annual male and female live births were obtained from a World Health Organization Mortality database for 1966±16years.
RESULTS: This study analysed a total of 58922297 births over the period 1950-82 (M/T 0.5146). The 1966 dip in total births was associated with a highly significant spike in M/T in the same year of 0.5184 (p<0.0001). 1965 and 1967 showed significant reductions from baseline M/T (p<0.0001). The periods immediately before and after 1965-67 showed higher baseline M/T (p≤0.008).
DISCUSSION: Sex blind strategies (contraception and abortion) have been shown to be responsible for the decline in total births. The increase in M/T in 1966 with a significant decline in M/T in 1965 and 1967 may be due to deliberate misattribution of birth year for female babies. M/T follows a U-shaped regression on cycle day of insemination. The higher M/T just before and after 1965-1967 may be due to higher coital rates in couples attempting to have a baby outside the Fire Horse year.